linux-kernel - Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET

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Message-ID: <20190807180033.lucbhob6wfunp5xf@wittgenstein>
Date:   Wed, 7 Aug 2019 20:00:34 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID

On Wed, Aug 07, 2019 at 04:26:10PM +0200, Oleg Nesterov wrote:
> On 08/06, Adrian Reber wrote:
> >
> > +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
> >  {
> >  	struct pid *pid;
> >  	enum pid_type type;
> > @@ -186,12 +186,35 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> >  		if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> >  			pid_min = RESERVED_PIDS;
> >  
> > -		/*
> > -		 * Store a null pointer so find_pid_ns does not find
> > -		 * a partially initialized PID (see below).
> > -		 */
> > -		nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> > -				      pid_max, GFP_ATOMIC);
> > +		if (set_tid) {
> > +			/*
> > +			 * Also fail if a PID != 1 is requested
> > +			 * and no PID 1 exists.
> > +			 */
> > +			if ((set_tid >= pid_max) || ((set_tid != 1) &&
> > +				(idr_is_empty(&tmp->idr)))) {
> 
> too many parentheses ;) this is purely cosmetic, up to you, but to me
> 
> 			if (set_tid >= pid_max ||
> 			   (set_tid != 1 && idr_is_empty(&tmp->idr))) {
> 
> looks a bit more readable.
> 
> 
> > +				spin_unlock_irq(&pidmap_lock);
> > +				retval = -EINVAL;
> > +				goto out_free;
> 
> This doesn't look right, you need idr_preload_end() before goto out_free.
> 
> But I'd suggest to simply do
> 
> 			nr = -EINVAL;
> 			if (set_tid < pid_max &&
> 			   (set_tid != 1 || idr_is_empty(&tmp->idr)))
> 				nr = idr_alloc(&tmp->idr, NULL, set_tid,
> 						set_tid + 1, GFP_ATOMIC);
> 
> 			...
> 
> this is more robust.

That all sounds reasonable to me.